Depending on the nature of the interactions between individual particles, colloids will form a surprising variety of morphologies. For strongly attracting systems, for example, they will irreversibly aggregate into fractal structures. In the opposite case of strongly repelling particles, similar systems will undergo an entropically-driven disorder to order transition forming colloidal crystals whose structure depends on the range of the repulsion. One of the technologically most compelling properties of these crystals is their ability to diffract visible light, a direct result of the length scales associated with these systems. Because of this characteristic and the ability of colloids to self-assemble, a number of researchers have attempted to take advantage of their optical properties for photonic applications, including photonic band gap materials and diffraction-based sensors.
A number of significant limitations however have slowed the development of colloidal crystal-based devices. The first is the difficulty in uncoupling particle-particle interactions, which is conventionally altered by modifying colloidal surface chemistry or solution properties, from the resulting crystal structure. Second, the structures that are created are not reversible over convenient time scales; colloidal crystals can take weeks to form. In addition, once formed, these crystals can be exceedingly fragile. Because they are typically created through long-range Coulombic interactions they can have lattice spacings several times their diameter and any shear can destroy them. Attempts to create stable crystals whose spacings can be varied with time or condition have hitherto focused on locking in the structure using a matrix that can expand or contract in response to an environmental stimulus.
For many desired applications however, the ability to reversibly create crystals is desirable. Applications such as color displays in which individual pixels must be switched is one example. To date, efforts to create reversible crystals have focused on two-dimensional systems in the context of electrodeposition, where it has been shown that applied AC or DC electric fields will induce ordering in two dimensions. The fields induce a strong attraction between particles causing a rapid aggregation into two-dimensional ordered domains. The effective interactions are due to electro-kinetic flows induced by the applied electric fields and the presence of the colloids. In the DC field case, the effect is dominated by electroosmosis and in the AC field case electrohydrodynamics dominate.
A general and efficient crystallization method is also desired in the field of protein chemistry and molecular modeling. This need is particularly acute due to the completion of the human genome project which has opened up new methods for understanding the fundamental biological processes of life. After determining all the genetic codes, the primary task of the post-genomic era is to understand the functions of the gene products—namely proteins—in cells and organisms. Proteins carry out essential functions in living organisms, and protein malfunction leads to a number of human diseases. A better understanding of the function of cellular proteins will therefore lead to the development of effective approaches for disease prevention and cure.
There are currently many approaches for studying the biological function of cellular proteins. One of the most powerful involves relating three-dimensional protein structure to its function. Protein structure determination can provide direct insight into how the protein works in a cell. Crystal structures at atomic resolution not only provide insight into the biological functions of proteins, but also allow the structure-based drug design of inhibitors or stimulators for regulating the functions of target proteins with known structures. As a result, universities and pharmaceutical companies are investing heavily in structural biology to determine the crystal structure of biologically important proteins.
Currently the most critical step in the determination of protein structure is the creation of a protein crystal in order to collect x-ray diffraction data. Protein crystallography has now advanced to the stage that, as long as a diffracting protein crystal is available, the structure for that protein can be determined. Therefore, the current bottleneck in determining protein crystal structure lies within the protein crystallization step.
Currently, there is no perfect methodology for growing protein crystals. Prediction of crystallization behavior is difficult because each protein behaves differently, requiring extensive efforts to screen hundreds and even thousands of conditions for a particular protein before it can be determined if the protein is crystallizable. Normally, the success rate of obtaining protein crystals is approximately 20-30%. Even for those proteins that produce crystals, months or even years of hard work can be required. Thus, a general and efficient method to crystallize any protein or any class of proteins would revolutionize structural biology and molecular medicine. Such a method would dramatically speed up the rate of structural determination of important protein targets and greatly reduce the costs associated with the crystallization process.